One of the steps in semiconductor fabrication is that of defining shapes and features in either semiconductor substrates, in films or layers formed on substrates, or in photomasks that will be used in photolithographic processes to transfer the shapes and features onto such substrates, films, or layers.
An electron beam, also called e-beam, may be used to generate such shapes by directly writing into a photo sensitive coating in which the shape is to be transferred. Other methods or systems of transferring the shapes include optical projection e-beam, projected ion beams, and X-ray lithographic systems. The advantage of electron beam over these other systems is that the e-beam system doesn't require photomasks, provides high resolution, and provides good level to level registration capability. The disadvantage of an e-beam system is that it is basically slow and is dependent on the number, complexity, and size of the shapes to be written. For most applications, photomasks are used to ensure consistent quality and imaging fidelity. The production of these photomasks is accomplished using similar direct-write technologies, where an energized beam (generally e-beam, optical, or ion) is used to form a pattern into a photo sensitive layer applied upon the photomask substrate. This latent image is then developed into freestanding structures which are used during a pattern transfer etch to transfer the image to the underlying films, which include, but are not limited to Chrome, Chrome oxide, Molybdenum silicide, quartz, and carbon.
Prior to development of the photoresist, so called chemically-amplified resists need to be processed through a thermal cycle to complete the chemical polymerization or depolymerization that results from the exposure of the photoresist to the energized beam. After a work piece such as a semiconductor substrate, wafer, or photomask, has been exposed by the patterning tool, it is sent to another tool or station, called the post-expose bake, PEB, station, where this thermal cycling occurs, generally between 85 and 200 degrees C., for a duration of, typically, between 30 seconds and 1 hour.
Post-expose bake time at the PEB station is usually less than the direct write time of the e-beam tool, typically by an order of magnitude. Thus, while a work piece, is in an e-beam system which has a separate, dedicated PEB station attached to each of its one or more e-beam patterning tools, the PEB station, most likely, will have already finished baking the previously patterned work piece and will be sitting idle waiting for the next work piece. This leads to under utilization of the post-expose bake station.
One way to resolve this problem is to configure the lithographic system with multiple e-beam patterning tools all connected to a single PEB station, thus yielding improved tool utilization, lower costs, and potentially improved product quality.
However, chemically amplified photoresists, used, in conjunction with high current e-beam tools, generally experience a degradation in resolution proportional to the time that passes between pattern generation and post-expose bake, called post-expose delay, or PED, time. This degradation, combined with the situation that different work pieces often require different patterning times and a lack of automated means of scheduling the writing and transport of the work pieces to the PEB station, can result in a variation in PED time which, in turn, will impact the average critical dimension (CD) and the dimensional distribution of the shapes on the finished product. FIG. 1 illustrates the negative effects that longer post-expose delay times have on various feature types. Here, experimental data for three different classes of shapes, namely, line shapes, isolated shapes and nested shapes, is represented. The post-expose bake delay versus the mean to target (or offset from desired image size) for each data type is plotted. The graph in FIG. 1 clearly shows degradation in the mean to target for each data class as the post-expose bake delay time increases.